A commonly practiced method of forming plastic materials into a desired shape employs the use of a prefabricated mold or die. Examples of typical molding processes include, without limitation, compression molding, transfer molding, injection molding, blow molding, foam molding, casting, plastisol casting, foam casting, thermo-forming, extrusion molding and the like. In these types of molding processes, a moldable material is introduced into, over, under, around or through a mold or die.
Typically, at some time during most molding processes, the material contained within the mold or passing through the die is in the form of a liquid or softened solid. Then, through the use of pressure (positive or negative, depending on the particular molding process), the moldable material is forced to conform to the shape of the mold or die. After the liquid (or softened solid) material conforms to the shape of the mold or die, the material is cooled to a temperature below that which it will not disfigure once removed therefrom. Typically, this is a temperature below the material's temperature of solidification (i.e., crystallization or glassification depending upon the particular material being molded).
In molding processes wherein there is a change in the physical state of the material contained within the mold or die, it is imperative to know when the material is ready to be removed therefrom. For example, if the molded material is removed too early, its shape can be susceptible to disfigurement. On the other hand, if the material is held in the mold too long, this can have adverse affects on the physical properties of the final product, as well as, increase production time which, in turn, decreases production output.
Moreover, in these types of molding processes, it is also imperative to know the time it takes for the moldable material to go from a temperature where it is in the form of a liquid or a softened solid to a temperature where it is in the form of a hardened solid. For example, if the molded material makes this transformation too rapidly, there may not be sufficient time for its molecules to orient in the desired manner. On the other hand, if the molded material makes this transformation too slowly, this can also have adverse affects on the final product's desired physical properties, as well as, increase production time which decreases production output.
As can be seen from the above, in many molding processes, it is essential to monitor the temperature of the material contained within the mold or passing through the die. Typically, heat sensors, such as thermocouples and infrared sensing devices, are used for this purpose. However, notwithstanding the present popularity of employing such heat sensors, there are many problems associated therewith.
For example, in conventional molding applications, thermocouples couples are not in direct contact with the material being molded. Specifically, thermocouples are typically imbedded into a probe having a metallic membrane or sheath which is designed to isolate the thermocouple from the moldable material's surface. Therefore, since thermocouples do not directly contact any part of the material being molded, they cannot be used as a means for accurately identifying the material's temperature in non-isothermal conditions. This phenomena can create a thermal lag effect which, in turn, can result in large errors when comparing the material's monitored temperature to its "actual" temperature at any given point in time during the molding process.
On the other hand, with regards to the implementation of infrared sensing devices, they too are plagued with inherent shortcomings in addition to being very expensive. For example, such infrared devices can only be successfully employed while the material being molded is above 120.degree. C. Therefore, for many commercial polymers (polyethylene, polypropylene, polystyrene, etc.), as soon as the moldable material solidifies, this temperature sensing means is no longer adequate. Moreover, it is also common knowledge that such infrared sensing devices have a limited temperature range.
Another shortcoming of infrared sensing devices is their relative cost. For example, based on other conventional heat sensors used for this purpose, infrared sensing devices are very expensive. This shortcoming is compunded by the fact that if it is desired to sense the temperature at different points in the mold, it is necessary to use additional infrared sensing devices, thereby increasing costs.
Other general shortcomings of conventional heat sensors are that they are typically associated with the mold's or die's inside wall surface. Accordingly, they are designed to only monitor the surface temperature of the material being molded. Therefore, these devices are unable to accurately monitor the material's internal temperature during a molding process. Since a material's internal temperature can differ significantly from its surface temperature, structural defects can result if proper measures are not taken to compensate for this difference.
As seen from the above, conventional sensing devices are either limited or unable to accurately track the material's actual temperature at any given point in time during the molding process. This is due, in part, to inherent features associated with their use (e.g., time lag, heat loss, presence of membranes through which the temperature must be monitored, etc.). Therefore, a margin of error is typically associated with their use.
Notwithstanding these problems, such heat sensing devices are presently the temperature monitoring systems of choice. However, if a means existed which could accurately monitor the temperature of a material throughout the molding process, it could be used to plot a cooling profile for a particular mold or die set-up. This would significantly enhance quality control, as well as, maximize production output and control in molding processes and processes such as, for example, those recited in U.S. Pat. No. 4,469,649 which are designed to alter the theological properties of a moldable material. Moreover, the advent of such a means would also eliminate the problems associated with the use of conventional heat sensors since they would become obsolete or at the most complementary in the case of infrared for the higher temperature ranges. Accordingly, the industry would graciously welcome such an improvement.